The present invention relates to novel polyphosphazene polymers having crown ether and related podand side groups. The polymers of the invention are useful as solid solvents for ionic conduction.
The beginning of the research field of solid polymer electrolytes (SPEs) can be traced back in time to the early 1970s when Wright (Fenton, D. E.; Parker, J. M.; Wright, P. V. Polymer 1973, 14, 589) (Wright, P. V. Br. Polym. J. 1975, 7, 319) (Wright, P. V. J. Polym. Sci.; Polym. Phys. 1976, 14, 955) discovered the ionic conductivity of poly(ethylene oxide), (PEO), containing dissolved inorganic salts such as KSCN. In 1978 Michael Armand (Armand, M. B.; Chabagno, J. M. and Duclot, M. Second International Meeting on Solid Electrolytes, St Andrews, Scotland, 20-22 Sep., 1978, Extended Abstract) (Annand, M. B.; Chabago, J. M.; Duclot, N. J. in Fast Ion Transport in Solids; Vashishta, P.; Mundy, J. N.; Shenoy, G. K. Eds. North Holland. 1979, 131) was the first to suggest the use of poly(ethylene oxide) as a solid solvent for rechargeable high energy storage batteries. Since then the field has rapidly grown to encompose a world wide range of research involving organic synthesis, polymer chemistry and physical chemistry, especially electrochemistry.
Poly(ethylene oxide) has, over the last 20 years, been extensively studied as an ionic conducting material, as shown in Chart 1 below. ##STR1##
The oxygen atoms contained in the backbone of PEO can datively coordinate to metal cations and thus form a variety of salt complexes with metal salts such as lithium triflate (LiSO.sub.3 CF.sub.3). The coordinating ability of PEO also facilitates ion-pair separation of disolved salts, and hence allows conduction of the free ions to occur. However, the level of conductivity also depends on the degree of crystallinity and molecular flexibility of the polymer system. A flexible polymer will allow high ion mobility and hence promote high ionic conductivity. The presence of crystalline domains in the polymer will impede ion mobility acting as blocks to the paths of ions and reducing the polymer flexibility. The three basic property requirements for polymers to act as solid polymer electrolytes are as follows:
(a) the polymer should exhibit a low glass transition temperature (T.sub.g), PA1 (b) the polymer should be amorphous and PA1 (c) should possess cation or anion coordinating sites to assist in the process of salt solvation and ion-pair separation.
Poly(ethylene oxide), PEO, (1) has a crystalline melting temperature of 65.degree. C. and does not become wholly amorphous untill heated above 100.degree. C. Hence, PEO is only an effective ionic conductor at 100.degree. C. or above. At room temperature the conductivity is significantly reduced due to the presence of crystallinity. The room temperature crystalline behaviour of PEO is a major disadvantage and has prompted many workers to attempt to modify the properties of PEO with the aim of eliminating the presence of crystallinity (Foos, J. S. and Erker, S. M., J. Electrochem. Soc. 1987, 134, 1724) (Goulart, G.; Sylla, S.; Sanchez, J. V. and Armand, M. B. in Second International Symposium on Polymer Electrolytes; Scrosati, B. Ed., Elsevier, London, 1990, 99) (Giles, J. R. M., Booth, C. and Mobbs, R. H. Transport-Structure Relations in Fast Ion and Mixed Conductors, in Proceedings, 6.sup.th Riso International Symposium on Metallurgy and Materials Science; Poulsen, F. W.; Anderson, N. H.; Clausen, K.; Skaarup, S. and Sorensen, O. T. Eds. Riso, National Lab., Roskilde, 1985, 329. Clausen, S) (Craven. J. R.; Mobbs, R. H.; Booth, C. and Giles, J. R. M., Makromol. Chem. Rapid. Commn. 1986, 7, 81) (Nicholas, C. V.; Wilson, D. J.; Booth, C and Giles, J. R. M., Br. Polym. J. 1988, 20, 289) (Linden, E. and Owen, J. R., Solid State Ionics, 1988, 28-30, 994) (Gray, F. M., Solid State Ionics, 1990, 40/41, 637) (Passiniemi, P.; TakkumSki, S.; Kankare, J. and SyrjSmS, M., Solid State Ionics, 1988, 28-30, 1001) (Przyluski, J.; Wieczorek, W.; Florjanczyk, Z. and Krawiec, W. Second Interantional Synposium on Polymer Electrolytes (ISPE-2), Siena, Jun. 14-16, 1989, Extended Abstracts, 56).
In the mid 1980s poly[bis(2-(2-methoxyethoxy)ethoxy)]phosphazene, 2, shown in Chart 1 also known as MEEP was first synthesized and studied as a polymer electrolyte by Allcock and Shriver et al (Blonksky, P. M.; Shriver, D. F.; Austin, P. E. and Allcock, H. R., J. Am. Chem. Soc. 1984, 106, 6854). The polymer is amorphous and when containing dissolved LiSO.sub.3 CF.sub.3 or AgSO.sub.3 CF.sub.3 salts was found to exhibit room temperature conductivities of up to three orders of magnitude higher than poly(ethylene oxide), PEO. This behavior is attributed to the high polymer backbone flexibility (low T.sub.g, -84.degree. C.) and high solvation or coordinating power to cations of the etheric side groups. This material has since been extensively studied as a solid polymer electrolyte by numerous other workers.
An object of this invention was to further investigate the properties of new polyphosphazene electrolytes. In particular, the effect of side chain structure on the final physical polymer properties, such as T.sub.g, crystallinity, ionic mobility and conductivity were evaluated. The side chain structure can significantly influence the T.sub.g and the degree of polymer crystallinity, which in turn strongly influences the ionic conductivity of the polymer/salt complex system.